Use more optimal call to find_last_of().
Use more optimal call to find_last_of().

Signed-off-by: Henner Zeller <259ad50fabbfd39355f36ecd6c0afca541ea66ef@acm.org>

chipsalliance/yosys-f4pga-plugins,SymbiFlow/yosys-symbiflow-plugins,antmicro/yosys-symbiflow-plugins,antmicro/yosys-symbiflow-plugins,chipsalliance/yosys-f4pga-plugins,SymbiFlow/yosys-f4pga-plugins,SymbiFlow/yosys-symbiflow-plugins,SymbiFlow/yosys-symbiflow-plugins,SymbiFlow/yosys-f4pga-plugins,antmicro/yosys-symbiflow-plugins,SymbiFlow/yosys-f4pga-plugins